This invention relates to the field of cathode ray tube deflection amplitude control by switched mode circuit and in particular to the synchronization and timing thereof during the horizontal interval.
The control of current flow in an inductance, by use of a switched mode driven circuit, is well known. The inductance may comprise a transformer, a choke or a deflection winding, with current flow therein being controlled by a switching signal. A well known example of the control of current flow in an inductance is the correction of pincushion and trapezium display errors by the use of a switched mode driven diode modulator. The modulator may comprise a class D amplifier where the output signal is integrated and coupled to the deflection circuitry. Such a modulating device is modulated by a constant amplitude signal, having pulses varying in width responsive to the required correction characteristic. For example, in an east-west correction system the modulating pulse repetition frequency is chosen to be synchronous with the horizontal rate, with a width variation or modulation, being determined by a vertical frequency, trapezoidal or parabolic waveform. The switched modulator is coupled to an inductive load, such as a deflection circuit, thus at turn-on the conducted current progressively increases from zero. However, at device turn-off, the energy stored in the inductive load will result in a voltage transient. Circuit techniques are known to minimize the various undesirable effects of such inductive turn off transients. For example, in a video display it is well known to arrange the timing or phasing of the horizontal frequency pulse signal to be such that turn off transients resulting from inductive switching occur during the non-displayed video intervals, thus rendering the transient invisible. However, positioning the transient to occur during a horizontal blanking interval may result in interference with various television signal elements present therein. Furthermore, since the horizontal frequency pulse is width modulated by, for example, a vertical frequency signal, the resulting turn off transient will also vary in position during the blanking interval. Thus, the transient, though not displayed and hidden from view, will move in horizontal position and may interfere with the color reference burst and or the "back porch" clamping interval. Such interference with the reference color burst may result in hue or saturation errors which vary in response to the vertical rate correction signal. Interference present during the "back porch" interval may result in similar vertical rate variations in the black level of the displayed image.
The inductive switch off transient may be coupled into the video signal by a number of different paths. For example, the transient may couple to the video signal via a power supply bus or ground trace. The transient may radiate and couple into the video signal path prior to subsequent processing thus resulting in a perturbation or glitch in the video signal, offset in time from the actual switch-off instant. It is also possible for a radiated turn off transient to be coupled to multiple pick up points, thus resulting in multiple glitches, offset in time one from the other in proportion to the subsequent propagation and signal processing delays encountered by the video signal.